The subject matter disclosed herein relates generally to the superconducting magnets, and more particularly to a method of passively shimming a superconducting magnet in an imaging system.
Magnetic Resonance Imaging (MRI) systems typically include a superconducting magnet that generates a primary magnetic field within an imaging volume. Inhomogeneities in the primary magnetic field may be caused by manufacturing tolerances for the magnet, as well as equipment and site conditions. In operation, magnetic field inhomogeneities may distort the position information in the imaging volume and degrade the image quality. The imaging volume must have a low magnetic field inhomogeneity to produce high quality images.
Shimming is a known technique for reducing the inhomogeneity of the primary magnetic field. The primary magnetic field is essentially a large constant field with small inhomogeneous field components superimposed on the constant field. If the negative of the inhomogeneous components of the field can be generated, the net field can be made uniform and the magnet is then referred to as shimmed.
It is known to use active or passive shims for reducing the magnetic field inhomogeneity. Active shimming may be accomplished using resistive and/or superconducting shim coils to generate magnetic fields designed to cancel out the inhomogeneous field components. Known passive shimming is accomplished after the magnet has been installed in the imaging system. Specifically, after installation, a magnetic field is induced into the magnet to identify inhomogeneous field components. Passive shims that cancel out the inhomogeneous field components are then identified. The passive shims are then installed in a conventional shim tray that is located near, or inside, a gradient coil structure that generates the x, y, and z gradient magnetic fields used for MR imaging.
However, during operation, the conventional passive shims are in thermal contact with the gradient coil structure, e.g. in the warm bore area of the MRI imaging system. Specifically, pulsing the gradient coils results in heat generation due to joule losses. A portion of the heat generated is transferred to the passive shims causing an increase in the temperature of the passive shims. The increased temperature of the passive shims reduces the magnetization of the shim material and weakens the magnetic field that the passive shims produce, thereby causing an increase in the magnetic field inhomogeneity. As a result, the passive shims installed in the shim trays installed in the warm bore area are subjected to varying temperatures that affect the magnetic permeability of the passive shims and therefore may not, adequately compensate for magnetic field inhomogeneities during operation. Moreover, there may not be adequate room in the shim trays to install the passive shims required to compensate for magnetic field inhomogeneities during operation.